Imperial College Rock Library

Glossary: Pyroclastic Rock

    Pyroclastic rocks are volcaniclastic rocks formed by accumulation of pyroclasts (fragments) during explosive eruption. The volcanic products of acid and intermediate magmas are largely pyroclastics and comprise a significant proportion of volcanic deposits and edifices such as stratovolcanoes, they are typical products of plinian and sub-plinian eruption styles. Pyroclastic rocks are, however, also produced by basic magmas in particular those that interact with water to produce phreatomagmatic surtseyan and strombolian eruptions. Alkaline magmas may also be associated with voluminous pyroclastic products.
Pyroclastic rocks are classified according to the size and abundance of their pyroclasts. Tuffs are pyroclastic rocks dominated by ash (pyroclasts <2 mm), lapillistones are dominated by lapilli (pyroclasts 2-64 mm in size), and pyroclastic breccias and agglomerates are dominated by blocks and bombs (pyroclasts >64 mm in size). Deposits consisting of a mixture of ash and lapilli are termed lapilli tuffs, whilst high unsorted pyroclastics containing ash, lapilli and blocks are termed tuff breccias. The general term tephra is used to described the fragmental materials produced by volcanic eruptions.
Pyroclasts can be classified by type into: (1) juvenile or vitric (magma-derived clasts), (2) lithic clasts derived from country rocks, and (3) crystals. Juvenile clasts are further subdivided on the basis of their form and mode of emplacement. Shards are glassy (vitric) ash, achneliths are glassy droplet or tear-drop shaped lapilli, pumice and scoria are highly vesicular juvenile lapilli or blocks, and bombs are ballistically emplaced blocks and lapilli. Accretionary lapilli are a form of sub-spherical lapilli with a concentric layering. Lithic clasts within pyroclastic rocks are usually pre-existing volcanic rocks. The dominant pyroclast is often used as a prefix giving classifications such as vitric tuff, crystal tuff, pumice lapilli tuff etc. Pyroclastic rocks containing abundant juvenile clasts can be welded or unwelded.
Depositional features, such as stratification, grading, dune-forms, cross-lamination, and topographic extent can be used to interpret the mode of emplacement of pyroclastic rocks into airfall, surge, pyroclastic flow deposits. These terms should not be used in the primary lithological description of pyroclastic rocks since they are interpretive. The term ignimbrite has been used, in particular in the US, to describe any pyroclastic flow deposit, however, it is also applied specifically to a welded pyroclastic rock containing flattened lapilli or blocks of pumice that is produced by high temperature pyroclastic flows.
The term volcaniclastic is used as a general term to describe any clastic rock or deposit produced by volcanic eruptions irrespective of whether these have been reworked by surface processes. The term epiclastic is used to specifically denote volcaniclastics that have been reworked.

Related Terms

achnelith, acid rock, agglomerate, basic rock, ignimbrite, intermediate rock, lapilli, lapillistone, phreatomagmatic eruption, plinian eruption, pyroclastic breccia, pyroclastic flow, pyroclastic surge, strombolian eruption, tuff, volcanic bomb

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